Automatic computing density range coupled light integrator



United States Patent Clarence Samuel 0st 118 N. Mansfield Ave., MargateCity, New Jersey 08402 211 Appl. No. 781,229

[22] Filed Dec. 4, 1968 [45] Patented Nov. 24, 1970 [72] Inventor [54]AUTOMATIC COMPUTING DENSITY RANGE COUPLED LIGHT INTEGRATOR 23 Claims, 5Drawing Figs.

Primary Examiner .lohn M. Horan Assistant Examiner- Richard A.Wintercorn Attorney-Sherman Levy ABSTRACT: A photographic apparatus forproducing screened reproductions consisting of a camera or enla'rgersystem with a copy support and an easel for photosensitive material.Exposure lamps are provided to illuminate the copy. The exposure lampsare energized by a control circuit including a photo-electricallyoperated selectively settable light integrator which is reset from oneadjustable balancing arm of a bridge circuit. Another arm of the bridgecircuit includes a light-responsive resistor probe which can be placedat different portions of the copy to respond to the densities thereof.The balancing arm can be adjusted for balance at a value correspondingto a selected highlight density area of the copy, or to the screen rangeminus a selected shadow density area. to thus respectively cause theintegrator to provide in each case a corresponding amount of light fluxfrom the exposure lamps. The bridge circuit includes another balancingarm which can be automatically adjusted in accordance with the screendensity range capability, and still another balancing arm which can beset to represent excess density. which is automatically computed while aselected shadow density area of the copy is being observed by thelight-responsive resistor probe. The system includes a flash lampadjacent the easel, energized by another selectively settable lightintegrating circuit which is reset in accordance with the setting of thelast-named balancing arm so as to provide a flash exposure having anamount of light flux corresponding to the excess density of the copydensityrange relative to the screen density range capability.

Patented Nov. 24, 1970 3,542,470

Sheet 1 of3- INVENTOR CLARENCE. S. 051- K; ATTORNEY Patented Nov. 24,1970 Sheet INVENTOR CLARENCE S.

Patented Nov. 24, 1970 3,542,470

Sheet ,5 of 3 JUNCTION CABLE INVENTOR ATTORNEY AUTOMATIC COMPUTINGDENSITY RANGE COUPLED LIGHT INTEGRATOR This invention relates tophotographic apparatus. and more particularly to exposure controldevices.

A main object ofthe invention is to provide a novel and improvedcombination of photoelectric and electronic elements comprising aninstrument to be used in the photographic industry for automaticallycalculating the various exposures required and for then controlling thetime duration of these exposures according to the intensity of theexposing radiation, so as to deliver a precalculated integral of radiantflux.

The present invention will find its most useful application in GraphicArts photography, and more particularly in the photo techniques employedin producing screened negatives and positives frorn opticallyhomogeneous copy.

Those skilled in the artknow that there is a limitation in the abilityofthe halftone screen to reproduce a range of tones. As an example: ifthe tonal range of the copy to be photographed in terms of the ratio oflight intensity reflected from (or transmitted through, in case the copyis a transparency) the minimum and maximum density areas is 200 to l,and the screen is limited in its ability to reproduce a range of tonesin the ratio of 25 to l, then tones will be lost either in the minimumor maximum or maximum density areas of the copy, or in both areas,depending upon the magnitude of the basic detail exposure or exposuresgiven. In this example as in the majority of cases, the copy range isgreater than the screen range. If, as in the case above, these ratiosare expressed as common logarithms (density) then the copy and thescreen have a density range of 2.3 and 1.4 respectively, which leaves anexcess copy density range of .9 which cannot be accommodated by thescreen. 2.3 l.4= .9.

lt is well'known in the art that the screen range will be affected by anvariation in optical geometry that will change the umbra, P-umbradistribution (vignette characteristic) at the focal plane behind eachscreen opening. Various densities of magenta and yellow CC filters usedwith magenta contact screens will vary the screen range. Also, if theexposure is split, partly with and partly without CC filters, the ratioof the 2 exposures will change the screen range. Also, the screen rangewill be altered if the ratio between a no-screen or bump exposure, andthe detail screen exposure changes. If optical screens are used, anychange between the ratio of stop diameter divided by cameraextension toscreen opening divided by screen distance, will alter the vignettecharacteristic and thus change the screen range. It is also known thatchanges in processing conditions which include time, temperature andchemistry, will alter the screen range by affecting the of the developedphotographic material.

It is very difficult to manipulate the factors affecting screen rangementioned above with any degree of predictable effect. However, it ispossible to hold a predetermined set of these factors constant and varya flash or fogging exposure, which lengthens the screen range andfurthermore can be controlled quite precisely.

' 'It should be noted here that the basic flash exposure is that amountof overall fogging exposure to the photographic material employed withthe screen in place, without the assistance of exposure to the copy,sufficient to produce, upon processing, the optimum geometric sizeshadow dot required for the process. If there is no excess density aflash is not required. However, if there is excess density, then theamount of flash required is determined by the amount ofexcess density.The following equation states that the actual flash exposure is equal tothe basic flash exposure times the quantity one minus the reciprocal often raised to the excess density power:

where F,, is the actual flash exposure, F,, is the basic flash exposure,and D, is the excess density.

It should be noted here that the equation above, which utilizes a flashexposure as a portion of the total basic flash, which portion is afunction of the excess density, in order to vary the screen range so asto equal the copy range is known in the art as the Controlled FlashTechnique and is widely used.

As is well known amongst those skilled in the art, the basic detailexposure is that exposure, or combination of exposures to the copy,regardless of copy or screen range, which will reproduce the minimumdensity area of the copy as the minimum geometric dot size required forthe process. If this minimum density area of the copy increases ordecreases in density, then the exposure should do likewise in the properproportion, so as to maintain this reproduced minimum dot sizerequirement uniform from reproduction to reproduction. The followingequation expresses this: where the actual exposure is seen to be equalto the basic exposure multiplied by ten raised to the minimum densitypower.

where E,, is the actual exposure, E,, is the basic exposure, and D,,,,,,is the minimum density.

The technique stated above of varying the detail exposure or exposuresfor the purpose of maintaining a uniform optimum dot size in thereproduction from the minimum density area of the copy will behereinafter referred to as Minimum Density Shift" in the discussion tofollow.

A certain small proportion of all copy being photographed in theindustry will fall into the category of having a range less than thescreen range established by the detail exposure or exposures employed.if this be the case, then of course a flash is not necessary. However,it would be desirable to reduce thev detail exposure or exposures by theproper amount, so that the maximum density area of the copy willreproduce the minimum geometric shadow dot required for the process. Theproper amount of this exposure required is expressed by the followingequation: where the actual detail exposure of exposures is equal to thebasic detail exposure or exposures times the reciprocal of ten raised tothe quantity, screen range minus maximum copy density power.

1 otsa-p where E,, is the actual detail exposure, 5,, is the basicdetail exposure, SR is the screen range, and D,,,,,, is the maximum copydensity.

The technique described above wherein the detail exposure or exposuresare shortened by the proper amount so as to reproduce a minimumgeometric shadow dot required for the process from the maximum densityarea of the copy, in which the copy range is less than the screen range,will be hereinafter referred to as Maximum Density Shift in thediscussion to follow.

When employing the maximum density shift equation, an excess screenrange at the highlight end of the copy tonal range will result, thusreproducing from the minimum density area of the copy a geometrichighlight dot too large for the process. In this case, if a singlescreened exposure was used, the effective screen range can be shortenedby employing a supplementary no-screen or bump exposure. if the originaldetail exposure was a combination of a no-screen andi screened exposure,then the proportion of no-screen to screened exposure should beincreased.

If the no-screen or bump exposure stated in the above ex amples is oftheproper amount, relative to the screened exposure, then the effectivescreen range can be made to match the that despite all theseramifications, the basic mathematics stated above are applicable to allthe various techniques.

It is the prime purpose of the present invention to provide a system orcombination of components comprising a machine or instrument so designedas to be able to gather density information from the copy to bephotographed, store, and then utilize this information in a computersystem -to solve the equations involvedand then automatically applythese mathematical solutions so as to modify previously programed basicexposures and then control the time duration of these exposuresaccording to the intensity of the exposing radiation, so as to deliver aprecalculated integral of radiant flux to the photosensitive material. 1

Further objectives of the invention will become apparent from thefollowing description and claims, and from the accompanying drawings,wherein:

FIG. 1 is a combination pictorial, block and schematic diagramillustrating the basicelements of a system according to the presentinvention and showing how they are interrelated and applied in carryingout the objects of the invention.

FIG. 2 is a diagram showing the relationship between the resistanceelements of potentiometers employed in the system of FIG. 1 and also themanner in which they are calibrated. FIGS. 3A and 38 together form aschematic wiring diagram of a practical device embodying the principlesand concepts of the present invention.

FIG. 4 is a diagrammatic view of a movable density probe unit employedin the present invention.

Referring to FIGS. 1 and 2, there can be seen associated with thepotentiometer 2 a scale logarithmically related to the potentiometersresistance element and calibrated in steps of 0.10 density units and thedensity position being shown at 20 percent of the total resistance ofthe potentiometer 2. This results in a scale range from infinite densitythrough 0 to approximately +.68D. This relationship is clearly shown inthe enlarged portion A ofFlG. 2, and is expressed by the followingequation:

where R is the scale value, D is density, and R is the resistance valueat the 0 position.

To simplify the following discussion, reference will be made to FIG. 1showing lamp 12 projecting beams 18 on areas 51, 52,55 and 60 andrespective reflected beams 20, 21, 19 and 61 striking photoresistivecell I. It should be understood that while FIG. 1 shows the use ofthepresent invention in its application to reflective copy, it also, withequal facility, can be applied to transparency copy in which case beams18 from lamp 12 would be impinging on areas 51, 52, 55 and 60, andrespective rays 20, 21, 19 and 61 will result after being transmittedthrough copy 17, etc.

Note that when switch 16 is in the left position a resistance bridgeisestablished consisting of photoresistive cell 1, potentiometer 2 andratio arm fixed resistors 5 and 6. The bridge is energized by battery22, and in its balanced condition it provides aO-deflection ofthepointer of meter 15.

The bridge is originally calibrated by setting potentiometer "2 tothe'density value of a known reflection of transmission densitystandard. Light from lamp 12 energized from battery 14 is then projectedon this density standard so that the light thus reflected or transmittedenters pbotoresistive cell 1. The bridge is then balanced by varying thelight output of lamp 12 by adjusting potentiometer 11. To those skilledin the art it willbe apparent that the optical geometry involved shouldbe rigidly fixed to assure reproducible accuracy.

After this original calibration it can be seen that if the light fromlamp 12 is allowed to impinge on other density areas to be measured.such as beam 18 to area 51 on picture 17, then the reflected ortransmitted beam 20 entering photoresistive cell 1 will change the cell1 resistance in inverse proportion to the intensity of the impingingbeam 20. Potentiometer 2 will then have to be adjusted to reestablishthe bridge balance as indicated by meter 15.

- Because the scale associated with potentiometer 2 is calibrated indensity units, it will indicate directly the density of the area underobservation. Furthermore, the resistance across potentiometer 2 willvary in inverse proportion to the intensity of the beam entering cell 1,or in direct proportion to the antilog of density (10") underobservation.

D =l0g10 1 Percent Transmission or Percent Reflection what its valuewould have been if beam 19 were impinging on an area of 0 density.

10- 2; 2X 10K ohrns= 20K ohms.

1n the following mathematical treatment, 1 represents the intensity ofthe beam and D represents the reflection or transmission density of thearea under observation. R represents resistance and V representsvoltage. The subnumbers 19, 20, 21 and 61 used with l, R and V willdenote the value of l, R and V when respective beams 19,20, 21 or 61 acton 1; also, the subnumbers 51, 52, 55 and 60, used with D, refer to thedensity of the areas under observation and the subnumber 18 used with 1refers to the intensity ofthe beam incident on the copy.

The subnumbers employed with R represent the resistances of therespective elements, for example, R is the resistance of element .2, Ris the resistance element 7, etc.

A movable density sensing optical probe unit 70 is employed, containingthe lamp l2, and the photosensitive element 1. This unit can be placedat any location on the copy 17 1 whose density is to be investigated.

1 =a constant after calibration K=a constant fixed by the opticalgeometry of 12, 1 and the surface of 17; therefore:

The resistance of 1, R1 is inversely proportional to its radiant input.

and R which makes 1 therefore R 10* 51; when the bridge is balanced:

Now because the sliders of R and R; are mechanically connected togetherand the slider position for zero resistance of R correspondsto theslider position for zero voltage of R;

with respect to bus 53, the voltage ratio at the slider of R; will be:

This states that the voltage at the slider of R, is directlyproportional to the antilog of the density under observation, because Vis aconstant reference voltage resulting from area 55, which has apractical density plateau of zero.

The upper right 'portion of FIG. 1 illustrates the integrator triggersystem which controls the amount of screened or screened plus no-screenexposure that is required to reproduce an optimum highlight dot in thereproduction from a prechosen area in the original copy (usually theminimum density area) Minimum Density Shift. This portion of P10. 1 mayalso be used toreproduce an optimum shadow dot in the reproduction froma prechosen shadow area in the original copy (usually themaximum densityarea) "Maximum Density Shift".

The emitter of transistor or the upper end of potentiometer 9 willalways be practically at the same potential as the slider ofpotentiometer 7. This is so because the series combination of the totalresistance of potentiometer9 plus the emitter-tobase resistance oftransistor 25 is high compared to the resistance of potentiometer 7,which it shunts.

With further reference to FIG. 1, it can be seen that the resistanceratio from the slider of R to bus 53, compared to the total R resistanceis the proportion of slider R; voltage applied to condenser 29 throughthe left contacts 39 of relay 37.

The potential charge on condenser 29 then becomes:

Vc EEEJL iKtofiBl) lt can now be seen that potentiometer 9, whichpreferably has an arbitrary linear calibration. is is the control whichestablishes the base magnitude ofthe potential applied to condenser 29.This base magnitude of potential is, of course, subject to variationaccording to the antilog of density D, under observation.

Still referring to'FlG. I, manually operated momentary exposure startswitch 43 is closed. This energizes relay coil '37 and holds contacts 40and normally open contacts 39 closed through the holding circuit andrelay power supply symbolized by block 33. Capacitor 29, which has beencharged to the potential of the slider on potentiometer 9, is nowshunted by the series combination of battery 28 and photoresponsivedevice 147. The polarity of battery28 opposes the charge'on condenser29, The photoresponsive device 47 should be preferably of the vacuum'photoemissive type which'has a substantially linear current vs. radiantenergy characteristic. Contacts 40 turn on thecamera lamps 45-45, whichin turn illuminate the copy 17 and the photoresponsive device 47.Capacitor 29 then discharges through photoresponsive device 47 at a timerate directly proportional to the intensity of impinging light fromlamps45-45. lt canbe seen that the initialpotential on capacitor 29,minus the potential at any instant, is analogous to the total radiantinput to photoresponsive device 47,-up to that instant. Therefore, ifthe discharge of capacitor- 29 is allowed to proceed until it is totallydischarged, its initial charge then is analogous to the radiant input topho- 'toresponsive-device'47and also to copy .17 being photog raphed.

This radiant energy integrator principle is explained in somewhatgreater detail in the present inventor's prior patents, U.S. Pat. No.2,408,576, U.S. Pat. No. 2,944,190 and U.S. Pat. No. 3,269,287;therefore, no further discussion of this principle will be given here.

Block 31 is preferably a practical infinite impedance type inputamplifier. such as a negative grid vacuum tube without grid return, or afield effect transistor. Extremely high resistances are important here,as any discharge path for capacitor 29, other than the battery 28 andphotoresponsive device 47, will interfere with the linearity, accuracyand stability of the integrator.

Block 32 symbolizes a stable trigger and voltage sensing circuit, havinglow hysteresis and able to accurately sense through amplifier block 31the precise instant that the potential of capacitor 29 reaches zero(during its discharge through photoresponsive device 47) and at thisinstantto trigger the holding circuit of block 33, thus deenergizingcoil 37, which enables contacts 40 to open, thus turning off lamps45-45, also returning contacts 39 to their normally closed condition,thus enabling capacitor 29 to charge in readiness for the next exposure.

A practical example of how the Minimum Density Shift is accomplished andemployed will now be discussed. Copy 17 is to be reproduced as ascreened negative in which it is desired to produce an optimum geometricsize highlight dot from the minimum density area 51. Beam 18 from lamp12 is allowed to impinge on area 51 (Minimum Density Area). Reflectedbeam 20 enters photoresistive cell ,1. Switch 16 is in the left handposition. The bridge is balanced, indicated by no deflection of thepointer on meter 15, by manually adjusting potentiometer 2. The sliderof R because it is mechanically connected to the slider on R now assumesa negative voltage with respect to bus 53 that will always be in directproportion to the antilog of density (10") under observation, in thiscase, area 51. This R slider voltage is applied to the base oftransistor 25 and appears across potentiometer 9.

The sensitivity of photoresponsive device 47 should be previouslyadjusted by the use of neutral density filters, irises,

graduated screen tints, or suitable devices to modulate the lightentering and impinging upon the cells photosensitive element. Thepurpose of adjusting the sensitivity of cell 47 is to secure a range ofexposures based on the intensity of lamps 4545, the sensitivity of thephotographic material being exposed, the aperture ratio, and otherfactors known to those skilled in the art. 'Oncethis sensitivity isestablished for the working conditions, it should remain fixed, andpotentiometer 9 is then utilized as a means of establishing baseexposures for the different types of screens and the various charactersof reproduction results desired. It does this by selecting a portion ofthe R; potential, which portion is applied to condenser 29 through theleft contacts of relay 37.

Copy 17 is placed on the easel of camera 50 and then photographed with asingle screened exposure or combination of a screened and no-screenexposure, or other exposures as desired by the operator.-The object ofthis first test is to find a setting for potentiometer 9 that willreproduce the optimum geometric size highlight dot required for theprocess from the area in the copy first observed (in this case theminimum den- Attention is now directed to HO. 1 and FlG. 2, and morespecifically to potentiometers 3 and 4 illustrated in the'circuit ofFIG. 1 and in the enlarged drawing of FIG. 2, which shows typicaldensity calibrations in scale relationship to the resistance elements ofthe Potentiometers. These relationships are expressed by the followingequations:

R otal It should be noted that the calibration of potentiometer 3 refersto the screen range .established by the exposure or exposures and otherphotographic parameters previously noted and expresses the ability ofthe screen to reproduce a particular range of tones unaided by a flashexposure. The calibration of potentiometer 4 expresses the excessdensity range in the copy not accommodated by the screen range.

A number of techniques for determining screen range are known to thoseskilled in the art. However, an automatic technique for settingpotentiometer 3 to the screen range (a prerequisite for furtheroperation of the instrument) will now be described.

The slider on potentiometer 2 is allowed to remain at its settingdescribed previously when area 51 was examined and recorded, with switch16 in the left-hand position. At this time, a base exposure (simple orcomplex) was established. sufficient to produce the optimum geometricsize highlight dot (required for the process) from this area. Switch 16is now positioned to the right, thus establishing a resistance bridgeconsisting of R R R and R Beam 18 from lamp 12 is then impinged on area60. allowing reflected beam 61 to affect photoresistive element 1. Area60 in copy 17 is chosen as that intermediate density area in which thecorresponding area of the negative reproduced (by the established baseexposure, in which area 51 was reproduced as an optimum highlight dot)the optimum geometric size shadow dot required for the process.

Potentiometer 4 is now set at the 0" no excess density position and thebridge is again balanced by adjusting the slider on potentiometer 3.

When the bridge is balanced:

therefore because 20 10 6 and 10 5 then Ra im total- 10 EHF H Also, asstated above 60 therefore 10 60 10 iU .%Z %P. or stated logarithmicallya 6 ex Because the excess density potentiometer R, was set at zero, itcan now be seen that balancing the bridge, by adjusting the screen rangepotentiometer R;,, will automatically set R;, at the density differencebetween the areas in the copy that reproduced respectively an optimumhighlight dot and an optimum shadow dot.

This difference is known to those skilled in the art as the screen rangeand is defined as the ability of the screen to reproduce a range oftones by any exposure or combination of exposures (as describedpreviously) but without the aid of a flash or fogging exposure.

Settings for screen range potentiometer 3 and base exposurepotentiometer 9 have now been determined and will remain set for theparticular screen, type of film used,

processing conditions, and character of reproduction required.

it should be noted that in a particular instrument it may be desirableto employ a multiple number of base exposure potentiometers 9, that maybe preset for multiple or complex exposure techniques. Any suitablemeans can then be used to switch from one to another as required. It canalso be seen that the antilog of density in the copy under observationwill then factor all these base exposures.

lt is now necessary, to determine the amount of base flash, or foggingexposure of flash lamp 48. sufficient to produce, upon processing film49, the optimum geometric size shadow dot required for the process.

Examination of H6. 1 will disclose that the top circuit con-. trollingbase exposures and comprising elements 7, 23, 25, 9,

29, etc. is similar to the bottom circuit comprising elements 8, 24, 26,10, 30, etc. which controls base flash exposures. The operation of thetop circuit has previously been described, therefore a discussion of theoperation of the bottom or base flash, circuit, being the same, will notbe repeated.

It is important that the sensitivity of photoresponsive device 48 bepreviously adjusted by the use of neutral density filters, irises,graduated screen tints, or any other suitable device to modulate thelight entering and impinging upon the cell's photosensitive element. Thepurpose of adjusting the sensitivity of cell 48 is to secure a range ofexposures based on the intensity of flash lamp 46, the sensitivity ofthe photographic material being exposed, the density of the screenemployed, and other factors known to those skilled in the art. Once thissensitivity is established for the working conditions, it should remainfixed. Potentiometer 10 is then utilized as a means of establishing baseexposures for the different types of screens and the various charactersof reproduction results desired. It does this by selecting a portion ofthe R,, potentiometer voltage, which portion is applied to condenser 30through the left contacts 41 of relay 38.

For the purpose of establishing the base flash, excess densitypotentiometer 4 is manually set at its infinite density calibrationmark. Now, because the sliders of R and R are mechanically connectedtogether and the slider position for zero resistance (infinite excessdensity, see FIG. 2) corresponds to the slider position for maximumvoltage with respect to bus 54 on R,,, then maximum voltage 'will beapplied across base flash potentiometer R Film 49 is placed in thecamera 50, covered with the desired screen, and then flash-exposed withlamp 46. The object of this test is to find a setting for potentiometer10 that will produce the proper exposure to reproduce, upon processing,the optimum geometric size shadow dot required for the process. Oncethis base flash setting for potentiometer 10 is determined, it remainsconstant for the screen, the type film used, and the processingconditions.

Attention is directed to the fact that the actual flash exposure givenis a proportion of the base flash established above,

R, can be seen to be:

l- IO eX, or simply Btotal This equation demonstrates that the portionof total voltage appearing across potentiometer 10 is equal to one minusthe reciprocal of the antilog of excess density.

It can be seen that the resistance ratio from the slider of R to bus 54compared to the total R resistance is the proportion of slider R,voltage applied to condenser 30 through the left contacts 41 of relay38. The voltage charge on condenser 30 then becomes:

lOslider Vcao= -V K, and because ltltotal V K 1 ex, then lOslider oso=(l- 1O ex) lfltotnl This shows that the integrator condenser potential Vis of course subject to variation according to the excess density in themanner indicated.

The above mentioned condenser 30 voltage (V then controls the amount offlash exposure received by film 49 from lamp 46 which is monitored byphotosensitive device 48 in a similar manner and as previously describedfor integrating condenser 29 in the upper section of FIG. 1.

An example of the actual use of the instrumentation will now bedescribed. It should be noted that base detail exposure potentiometer 9has been set as previously described for the type film, processingconditions, geometric size highlight dot required. etc. Also, base flashpotentiometer 10 and screen range potentiometer 3 have been set aspreviously described.

Switch 16 is placed in its left position; the minimum density area 51 ofcopy 17 is then observed by impinging beam 18 thereon and allowingreflected beam 20 to enter photosensitive device 1. Minimum densitypotentiometer 2 is then adjusted for bridge balance; then leavingpotentiometer 2 in its set position, switch 16 is then positioned to theright. Area 52, the maximum density area of the copy, is then observedby impinging beam 18 thereon, allowing reflected beam 21 to enter andaffect photosensitive device 1. At this time, excess densitypotentiometer 4 is adjusted for bridge balance.

flash exposure is now given to film 49 by of closure fswitch 44. Afterthe flash exposure is completed, the easel of camera 50 is placed in itsfocal plane position and film 49 is exposed to copy 17 by momentaryclosure of switch 43. It can be seen that these two exposures may begiven simultaneously if flash lamp 46 is positioned inside the cameraproper and photosensitive device 48 is allowed to exclusively respond tothe output of lamp 46.

lt should now be apparent to those skilled in the art that, although thepresent invention is complex in theory, it is basically simple in itsuse and application. It should be noted that when entering the minimumand maximum density areas of the copy, digital information is displayedon the dials. This information, however, is academic, and readout is notnecessary in the normal use of the instrument. Also, setting the dialsto digital information is not normally required. The operator merelyoperates the dials of the potentiometers 2, 3 or 4, as the case may be,until the pointer on meter 15 is centered.

Further operator convenience is available in locating the minimum andmaximum density areas of the copy more accurately than can be donevisually. This is accomplished by watching the pointer of meter 15 whilethe optical unit 70 comprising lamp l2 and photoresponsive device 1 isbeing moved from one area to another on copy 17. Maximum deflection ofthe meter 15 pointer to the right will indicate maximum density, andconversely to the left, minimum density.

The maximum density shift equation 5,, E,,*

is applied automatically in those special cases where the copy range isless than the screen range. In the normal procedure,

the minimum density of copy 17 is entered first by adjustingpotentiometer 2 with switch 16 in the left position and with the opticalsystem 12-1 observing the minimum density of the copy. Switch 16 is thenpositioned to the right and the optical system l2l is positioned on themaximum density area of copy 17. It will now be found that potentiometer4 reaches the zero excess density, or no flash, position before thepointer on meter 15 can be centered. This signals the operator that thecopy range is less than the screen range and also reduces the flash tozero, which is correct for this type of copy. The operator leavespotentiometer 4 at its zero position and then centers the pointer onmeter 15 by readjusting potentiometer 2 from its original setting. Thiswill shorten the detail exposure or exposures so as to place the maximumdensity area of copy 17 in the optimum shadow dot region ofthereproduction.

The following analysis demonstrates the validity of the aboveconclusions:

Note that D is entered on potentiometer 2 and represents the minimumdensity area of the copy. Also that D50 appears across photoresponsivedevice 1 as a resistance representing the maximum density area of thecopy. Therefore:

Dmax Dmin =Dsr+Dex In the demonstration above, Dex=0; thereforeDmin=DmaxDsr, or as anti-logs:

it has been explained previously that potentiometer 2, due to itslinkage with potentiometer 7, factors base exposures set onpotentiometer 9. It therefore can be seen that the expression10(DsrDmax) agrees with the E,, factor in the maximum density shiftequation.

It is known to those familiar with the art that increased noscreenexposure should be given in the special case above (or other techniquesemployed) in order to shorten the effective screen range thereby closingup the highlights caused by the decrease in the original detailexposure.

In FIGS. 3A and 38 there is shown a schematic diagram depicting aspecific practical embodiment of the present invention. A detaileddescription of this practical circuit is not necessary because thoseskilled in the electronics art can readily discern the interrelation ofits elements and can also understandably compare the practical circuitfunctions with the broad general block diagram ofFlG. 1.

Many modifications of the present invention are possible withoutdeviating from its scope and purpose, and of necessity it would makethis disclosure too lengthy to attempt to anticipate every possibletype. However, the following modifications are anticipated and willserve to illustrate expansions and variations of this invention withoutdeviating from its general scheme.

It would be advantageous in some aspects of the present invention toseparate the densitometer computer section comprising the elements 1,2,3,4, 5,6, 15, 16,22, 11, 12 and 14 from the rest of the instrument. Withthe instrument thus divided, all copy could then be densitometricallyanalyzed at a central location by one operator. An integral suitabledevice would then punch or marka card indicating the computed positionsof the potentiometers 2 and 4, which card would then be attached to thecopy. The cameras in the plant would be equipped with the integratorsand lamp control portions of the invention. The punched or-marked cardattached to the copy would then be utilized in suitable fashion toposition potentiometers 7 and 8, either by manual-visual or automaticmeans. l i

it may be more convenient in some applications of the present inventionto use a sampling technique-for exposing photo responsive devices 47 and48 by employing another lamp or lamps contained within the main cabinetof the instrument. The photoresponsivc device 47 or 48 would then beexposed to the light emitted'from the sampling lamp instead of thecamera lamps. A prerequisite for the feasibility of this technique isthat the camera lamp and corresponding sampling lamp be of the. samegeneral type, such as incandescent,

- etc., and thatboth be connected to the same electrical mains.

The output from-both the sampling and camera lamps would then vary inproportion to the voltage fluctuations of the isjencountered and densitometrically controlled and c omputed timing is indicated; this maybeaccomplished by substituting constant current means in placeofphotoresponsive devices 47 and 48.

Another variation of the present invention can be achieved by operatingpotentiometers 2-7 and 4-8 with motors con-- trolled by the signaloutput from the bridge circuit. This method would be more automatic thanthe manual-visual method described in the above disclosure.

A further expansion ofthe present invention would be in the use ofphotoresponsive device 1 (together with a continuous light attenuatorfor order-of-magnitude adjustment) as a means of directly reading thelight impinging upon the focal plane of an enlarger or camera system. Inthis way indirect measurement of the copy minimum and maximum densityareas and other areas, as the case may be, can be read andfed into thecomputer section of the instrument. Those skilled in the art willrecognize that this method has the advantage ofautomatic exposurecorrection for various reproduction scales.

While certain specific embodiments ofan instrument for automaticallycalculating and cont-rolling photographic exposures have been disclosedin the foregoing description, it will be understood that various furthermodifications within the spirit of the invention .may occur to thoseskilled in the art. Thereforeit is intended that no limitations beplaced on the invention except as defined by the scope of the appended.

claims.

l claim:

1. An apparatus for producing photographic reproductions fromcopyemploying means that utilizes the copy's densities to compute theexposures required and means to expose-the photosensitive material tosaid computed exposures, comprising a cameraor enlarger system havingthe usual supportfor copy and easel for photosensitive material, circuitmeans including a light responsive probe which can be positioned atdifferent areas of the copy to respond to said areas densities, saidcircuit means having settable light integrating means, copy exposurelamp means positioned to illuminate the copy, means energizing said copyexposure lamp means for a duration in accordance with the density of aselected area in the copy or to a computed combination of densities,circuit means to compute and set screen range and means for computingexcess copy density rangerelative to said screen range, and flash lampmeans located to illuminate said photosensitive material includingsettable light integrating means to energize said flash lamp means for aduration to provide an integrated flux in accordance with the saidexcess density range of the copy rela tive to the screen range.

2. An apparatus for producing screened photographic reproductions fromcopy employing means that utilizes the copys densities to compute theexposures required and means to expose photosensitive material to saidcomputed exposures, comprising a camera or enlarger system having theusual support for copy and easel for photosensitive material, circuitmeans including a light responsive probe which can be posiwhich the saidlight responsive probe had first been positioned, said circuit meanshaving further means for setting the density difference between thosedensities in the copy that reproduced respectively an optimum highlightdot and an optimum shadow dot without a flash exposure, corresponding tothe screen density range, and means in said circuit means to compute theexcess of the copy density range relative to the screen density rangeutilizing the previously selected and set highlight density, thepreviously computed and set screen density range and a selecte'dshadowdensity area in the copy'at which the light responsive probe ispositioned, flash lamp means located to illuminate said photosensitivematerial, and light integrating means to set a selected basic flashexposure, and means to energize said flash lamp means for a duration toprovide an integrated flux which is a portion ofsaid basic flashexposure in accordance with the excess of the density range of the copyrelative to the screen range, said integrated flash flux being theproduct of said basic flash exposure multiplied by the quantity oneminus the reciprocal of the antilog of excess density. I

3. The apparatus of claim 2, wherein ifthe copy range is less than thescreen range, said exposure lamp means will be energized for a durationin accordance with a modification of'the set basic exposure of saidlight integrating means, whereby the flux provided the copy will be theproduct of said set basic exposure multiplied by the reciprocal of theantilog of the quantity of said screen range minus the density of aselected shadow area of the copy at which the said lightresponsive probeis placed.

4. An apparatus for producing screened photographic reproductions fromcopy by densimetrically controlled and computed exposures comprising acamera or enlarger system having a support for copy and an easel forphotosensitive material to be exposed, a bridge circuit including in onearm thereof'a movable light responsive resistor which can be positionedat different areas ofthe copy to respond to their respective densities,said bridge circuit having a second arm'including an adjustable secondresistor for balancing the bridge circuit, selectively settable lightintegrating means, and copy exposure lamp means located to'illuminatethe copy, means energizing said copy exposure lamp means for a durationin accordance with the setting of said light integrating means modifiedby said second resistor, whereby the flux provided the copy can be inaccordance with a measured selected density of the copy or to a computedcombination of densities, said bridge circuit having a third armincluding an adjustable third resistor for balancing the bridge circuit,means to adjust said third resistor to a value at balance representingthe density difference between the two areas in the copy that producedrespectively an optimum. highlight and an optimum shadow dot in thereproduction without a flash exposure, said third resistors value thuscorresponding to the screen range, said bridge circuit having a fourthresistive arm, selectively settable second light integrating means toset basic flash exposures including flash lamp means located toilluminate said photosensitive material, and means to energize saidflash lamp means for a duration to provide a flux in accordance with thesetting of said second light integrating means modified by said fourthresistive arm.

5. The apparatus of claim-4, and wherein the meansto modify said basicflash exposure comprises the said adjustable fourth resistor in saidfourth arm for balancing the bridge circuit, means to adjust said fourthresistor to a value at balance corresponding to the excess density ofthe copy density range relative to the screen density range as computedby said bridge circuit utilizing said second resistor previously set tocorrespond to a selected highlight density area of the copy and saidthird resistorpreviously set to correspond with the screen range andsaid light responsive resistor "probe sensing a selected shadow area ofthe copy.

6. An apparatus for producing screened photographic reproductions fromcopy comprising a camera or enlarger system having a support forcopyandan easel for photosensitive material to be exposed, a bridgecircuit including in one arm thereofa movable light responsive resistorprobe which can be'positioned at different portions of. the copy torespond to their respective densities, said bridge circuit having asecond arm including an adjustable second resistor for balancing thebridge circuit. light integrating means, means to control the fluxresulting from said light integrating means in accordance with theadjustment of said second resistor,-copy exposure lamp means located ina positionto illuminate the copy, means energizing said copy exposurelampmeans for a duration in accordance with said light integratingmeans, whereby the flux provided can be in accordance with thedensity ofa selected highlight area of the copy or a computed combination ofdensities, said bridge circuit having athird arm including an adjustablethird resistor for balancing the bridge circuit, means to adjust saidthird resistor to a value at'balance representing the density differencebetweenthe two areas in the copy that reproduced respectively an optimumhighlight dot and an optimum shadow dot in the reproduction withoutaflash exposure, said third resistors value thuscorresponding to thescreen range, said bridge circuit'having a fourth resistive arm, flashlamp means located in a position to illuminatesaid photosensitivematerial, and means to energize said flash lamp means fora duration to.provide a flux inaccordance withthe excessdensity of the densityrange ofthe copy relative to the screen range.

7. The apparatus ofclaim 6, and wherein the means to energize said flashlamp means comprises anadjustable' fourth resistorin said fourth arm forbalancing the bridge circuit, means to adjust said'fourthresistor to avalueat balance cor responding to the excess density of the copyutilizing said probe resistor, second light integrating means,'means tocontrol the setting of said second light integrating'means in accordancewith the adjustment of said fourth resistorpand means to energize saidflash lamp means in accordance with the setting of said second lightintegrating'means.

8. The apparatus of claim 6, and wherein said integrating means andexposure lamp energizing means comprises a capacitor. means to chargesaid capacitor to a voltage in accordance with the adjustment of saidsecond resistor at balance, a discharge circuit including acurrentsource and a photocell exposed to said exposure lamp means, meansto simultaneouslyconnect the capacitor to saiddischarge circuit I andenergize the exposure lamp means, and means-to terminate energization ofsaid exposure lampmeans when the capacitor becomes substantiallycompletely discharged.

9. The apparatus of claim 8, and wherein the means to charge saidcapacitor comprises a source of current, a transistor havingitscollector and emitter connected across said last-named source with animpedancein circuit therewith, means to apply a portion of the potentialof said last-named sourccof current controlled in accordance with thesetting of saidadjustahlc second resistor to thc'base of the transistor.

and means connecting at least a portion of said impedance across thecapacitor.

it). The apparatus of claim 9. and wherein the means to apply saidportion of the potential of the last-named source of current to thetransistor base comprises a potentiometer connected across saidlast-named source with its slider connected to said base, and meansmechanically or remotely coupling said slider to said adjustable secondresistor.

H. The apparatus of claim 8, and wherein the means to simultaneouslyconnect the capacitor to said discharge circuit and energize theexposure lamp means comprises a control relay having respective pairs ofcontacts in circuit with said lamp means and said capacitor anddischarge circuit, a holding circuit connected to said relay andlatching said relay energized. and means to momentarily energize saidrelay so as to render said holding circuit effective, and wherein themeans to terminate .energization of said exposure lamp means comprisesmeans to release said holding circuit responsive to substantiallycompletedischarge of said capacitor.

l2. Theapparatus ofclaim 7, and wherein said second light integratingmeans and flash lamp energizing means comprising a capacitor, means tocharge said capacitor to a voltage in accordance with the adjustment ofsaid fourth resistor at energize the flash lamp means, and means toterminate energization of said flash lamp means when the capacitorbecomes substantially completely discharged.

13. The apparatus of claim 12, and wherein the means to charge saidcapacitor comprises a source of current, a transistor having itscollector and emitter connected across said last-named source with animpedance in circuit therewith, means to apply a portion of thepotential of said last-named source controlled in accordance with thesetting of said adjustable fourth resistor to the base ofthe transistor.and means connecting at least a portion of said impedance across thecapacitor.

14. The apparatus of claim 13, and wherein the means to apply saidportion of the potential of the last-named source of current to thetransistor'base'comprises a potentiometer connected across saidlast-named source with its slider connected to said'base, and meansmechanically or remotely coupling said slider to said adjustable fourthresistor.

15. The apparatus of claim .12. and wherein the means to simultaneouslyconnect the capacitor to said discharge circuit andenergize the flashlamp means comprises a control relay having respective pairs of contactsin circuit with said flash lamp means and said capacitor and dischargecircuit. a holding circuit connected to said relay and latching saidrelay energized, and means to momentarily energize said relay so as torender said holding circuiteffective, and wherein the means to terminateenergization of said flash lamp means comprises means to release saidholding circuit responsive to substan- 'required comprising circuitmeans having a plurality of parameters which can be adjusted, saidparameters including means responsive to the density of the copy, meansto establish a base exposure which can be modified in accordancewith theantilog of the density under observation, means adjustable in accordancewith a density difference corresponding to the Screen range, and meansto establish a flash exposure in accordance with the excess density ofthe copy relative to the screen range.

18. In an apparatus for'producing screened photographic reproductionsfrom copy. a device to compute the exposures required comprising circuitmeans having a portion which can he current-balanced and having aplurality of parameters which can be adjusted for balance. saidparameters including means responsive to the density of the copy, meansto establish a base exposure which can be modified in accordance withthe antilog of the density under observation, means a adjustable inaccordance with a density difference corresponding to the screen range,and means to establish a flash exposure in accordance with the excessdensity of the copy relative to the screen range.

19. The apparatus of claim 17, and wherein the means responsive to thedensity of the copy comprises movable light sensitive transducer meanswhich can be placed at any selected area ofthe copy. a

20. The apparatus of claim 17, and wherein the means to establish thebase exposure comprises means to develop a voltage in accordance withthe antilog of the density under observation, exposure timing means, andmeans to adjust said timing means in accordance with said voltage.

21. The apparatus of claim 17, and wherein said flash exposure isproportional to one minus the reciprocal of the antilog of said excessdensity.

22. In an apparatus for producing photographic reproductions from copy,a device to compute the exposures required comprising circuit meanshaving a plurality of parameters which can be adjusted to provide adefinite current condition in a portion of said circuit means, saidparameters including means responsive to the density of the copy. meansto establish a basic detail exposure which can be modified in accordancewith the antilog of a density under observation, means to establish abase flash exposure, and means to modify said base flash exposure inaccordance with one minus the reciprocal of the antilog of the excessdensity of the copy relative to the screen range.

23. The apparatus of claim 22, and means to modify said basic detailexposure according to the equation:

Ea Eb- 1 where E is the actual detail exposure;

E,, is the basic detail exposure;

SR 'is the screen range; and

D is the density ofa selected shadow area of the copy.

